1. Field of the Invention
The present invention relates to the coloration of solid-state electrochromic oxide devices.
2. Description of the Prior Art
Electrochromism is the property of a material or system to change color or form color centers reversibly in response to an applied potential. Electrochromism occurs when a material exhibits an intense color change when subjected to an electric field of sufficient strength. The intensity of coloration is proportional to the applied field and the length of exposure.
Several theories have been advanced to explain the solid-state electrochromism phenomenon in transition metal oxides. The effect was originally thought to result from electrons injected into the metal oxide and trapped at oxygen vacancies or similar defect centers. Others interpreted the coloration action as an electrochemical extraction of oxygen with the colored product being stoichiometric. Still others proposed that coloration is brought about not by insertion of hydrogen atoms, but by redistribution of already incorporated hydrogen atoms from optically inactive sites to optically active sites within the transition metal oxide film. The most commonly accepted explanation for the electrochromic effect in transition metal oxides appears to be the simultaneous injection of electrons and protons, or positive monovalent ions such as Li.sup.+, Na.sup.+, and the like, into the material which forms a blue hydrogenated bronze.
It is generally recognized that there are a number of metal oxides which display cathodic coloration, i.e., coloration which commences at the cathode terminal of the device. Following is a partial listing of the materials known to display this phenomenon upon the application of an electric field.
(A) WO.sub.3 PA1 (B) MoO.sub.3 PA1 (C) WO.sub.3 /MoO.sub.3 PA1 (D) V.sub.2 O.sub.5 PA1 (E) Nb.sub.2 O.sub.5 PA1 (F) TiO.sub.2 PA1 (G) W.sub.9 Nb.sub.8 O.sub.47 PA1 (H) WO.sub.3 /ReO.sub.3 PA1 (I) WO.sub.3 /Au
While each of the oxides listed above displays cathodic coloration, tungsten trioxide (WO.sub.3) is known to have optical and electrical properties that are best suited for the fabrication of thin film devices. The physical properties of this material have been extensively studied in recent years due to its ferroelectric characteristics and its unusual ability to form tungsten bronzes when positive monovalent ions are incorporated into its lattice.
Electrochromic behavior of an evaporated WO.sub.3 thin film was first observed in an arrangement of two closely spaced metal terminals secured to a film. Blue coloration was generated adjacent to the cathode which migrated laterally toward the anode under an applied field of about 1.times.10.sup.4 V/cm. The value of the field intensity required to actuate coloration in devices of the relevant type is known to limit the interelectrode coloration distance to only a few millimeters. Consequently, the application of a voltage for about 0.5 to 2.5 hours is required to complete the coloration process.
In view of these considerations, researchers have focused on a planar version of the electrochromic device described above, i.e., a device which has a plurality of layers. It is generally recognized that lower voltages are required to actuate coloration in devices of planar configuration. However, in such configurations, coloration occurs perpendicularly through the transition metal oxide layer and is particularly sensitive to fabrication and thin film deposition conditions. That being the case, pinholes, defects, and morphological irregularities introduced during the fabrication process lead to parasitic currents which often reduce the efficiency of coloration and the electro-optic reproducibility of the device.